Steam-assisted crystallization of TPA+-exchanged MCM-41 type mesoporous materials with thick pore walls

Materials Research Bulletin 47 (2012) 1774–1782

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Steam-assisted crystallization of TPA+-exchanged MCM-41 type mesoporous materials with thick pore walls Hong Li Chen, Kun Zhang, Yi Meng Wang * Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, Shanghai 200062, PR China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 15 June 2011 Received in revised form 1 March 2012 Accepted 16 March 2012 Available online 28 March 2012

Hierarchical MCM-41/MFI composites were synthesized through ion-exchange of as-made MCM-41 type mesoporous materials with tetrapropylammonium bromide and subsequent steam-assisted recrystallization. The obtained samples were characterized by powder X-ray diffraction (XRD), UV–vis diffuse reflectance spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis, FT-IR, 1H–13C CP/MAS and nitrogen adsorption–desorption. The XRD patterns show that the MCM-41/MFI composite possesses both ordered MCM-41 phase and zeolite MFI phase. SEM and TEM images indicate that the recrystallized materials retained the mesoporous characteristics and the morphology of as-made mesoporous materials without the formation of bulky zeolite, quite different from the mechanical mixture of MCM-41 and MFI structured zeolite. Among others, lower recrystallization temperature and the introduction of the titanium to the parent materials are beneficial to preserve the mesoporous structure during the recrystallization process. ß 2012 Elsevier Ltd. All rights reserved.

Keywords: A. Structural materials B. Chemical synthesis C. X-ray diffraction D. Microstructure

1. Introduction Mesoporous material M41S, possessing high surface area and tunable pore size, has attracted considerable attention because of its potential applications in catalysis, separation, and sorption for very bulky molecules. MCM-41, with a well-ordered, hexagonal array of parallel pore channels is undoubtedly the best known and most widely studied material in this family. Its pore diameter could vary from 1.5 to 10 nm by using post synthesis approach, adjusting surfactant concentration in the presence of small cations, changing the alkyl chain length of the templating surfactants, or introducing organic swelling agents [1–3]. Unfortunately, M41S-type materials are liable to collapse when mechanically compressed or when exposed to boiling water or steam due to the amorphous nature of their frameworks and thin walls [4,5]. Furthermore, the lower hydrothermal and mechanical stability and reduced acidity of M41S materials compared to zeolites limit their practical applications. In contrast, zeolites with crystalline framework, high stability and strong acid sites are widely used in petro-chemistry and fine-chemical synthesis and show size- and shape-selectivity [6–8]. Zeolites, however, often suffer from diffusion limitation due to small size of the channels (less than 0.8 nm) and cavities (typically < 1.5 nm).

* Corresponding author. Tel.: +86 21 62232251; fax: +86 21 62232251. E-mail address: [email protected] (Y.M. Wang). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2012.03.029

Obviously, mesoporous zeolites, with combined advantages of both mesoporous materials and crystalline microporous zeolites, are extremely desirable for catalysis and adsorption due to the possibility of overcoming the diffusion and mass transport limitation [9,10]. Successful attempts aimed to combine the properties of mesoporous materials and zeolites have been undertaken. One means is to synthesize hierarchical zeolites, such as mesopore generation into the zeolitic crystals via hard-casting template techniques, the assembly of zeolite nano-particles, steaming treatment and chemical leaching approaches (desilication and dealumination) [10–14]. However, the complexity of the template preparation has limited the industrial application of hard-casting techniques; and mesopores formed during leaching were predominantly cavities in the zeolite crystals rather than cylindrical pores connecting the external surface with the interior of the crystal [15] thus the intracrystalline diffusion was not significantly enhanced. The other is to introduce zeolitic ordering into the mesopore walls, where amorphous walls of mesoporous silicates (e.g. MCM-41 or SBA-15) are converted to a partially zeolitic product and the ideal material may possess a ‘zeolite-type’ wall structure while maintaining the mesoporous ‘super-structure’ [16–25]. This kind of materials will be given more attention because it is provided with several important traits, such as bimodal pore system, more active sites, improved mechanical, thermal and hydrothermal stability and others, making them more suitable for large molecules’ adsorption and catalytic reaction. Unfortunately, it is difficult to avoid the formation of simple mixture of bulk zeolite and mesoporous materials during the

H.L. Chen et al. / Materials Research Bulletin 47 (2012) 1774–1782

transformation process from amorphous walls to partial crystallized ones. Moreover, material with a ‘zeolite-type’ wall is mostly lack of long-ordered microporous structure, which will reduce the crystallinity and limit its size- and shape-selectivity. To solve the phase-separation problem of the zeolite and mesoporous phases, Ryoo et al. directly synthesized zeolite MFI nano-sheets with thickness of only 2 nm and mesoporous MFI and LTA zeolite with tunable mesostructure using specifically designed bi-functional cationic surfactants [26,27] and amphiphilic organosilanes [28] as single template, respectively. However, both bifunctional cationic surfactants and amphiphilic organosilanes are complex and specifically designed. Most works devoted to dualtemplate method to obtain hierarchical micro- and meso-porous materials [29–31], where zeolite structure was directed by small structure-directing agents (SDAs) and mesoporous structures formed according to the supramolecular templating mechanism of the surfactant micelles. Nonetheless, these two different templating systems worked in a competitive manner when they were directly mixed together and thus the obtained materials mostly were the physical mixtures of amorphous mesoporous material and bulky zeolite. Some efforts have been done using multi-step synthesis strategies to ensure the surfactant and SDAs work in a cooperative rather than a competitive manner and, to some extent, avoided the formation of simple mixture of and bulky zeolite and amorphous mesoporous material. In the first step, mesoporous parent materials were prepared using large molecular surfactants as template; secondly, small SDAs were positioned at the interporous surface of mesoporous materials, which then induced partial recrystallization of pore walls during thermal process. Yue et al. [32] reported that MCM-41/ZSM-5 was fabricated through impregnating a structure-directing agent into as-synthesized MCM-41 followed by dry-gel conversion (DGC), where amorphous silica was transformed to zeolite crystal and the composite suffered the complete collapse of original ordered mesostructure of parent materials. van Bekkum et al. [33] reported that partial recrystallization of MCM-41 could be induced by impregnation of tetrapropylammonium hydroxide (TPAOH) followed by a hydrothermal treatment. However, alkaline conditions during the impregnation and hydrothermal treatment inevitably led to the collapse of the MCM-41 framework. To avoid rapid collapse of the MCM-41 framework, Kloetstra et al. [24] applied glycerol as the medium for recrystallization. Unfortunately, the formation of ZSM-5 crystallites was inhibited, though the mesostructure of MCM-41 was better retained. Another reason behind the damage of MCM-41 mesostructure during the recrystallization process was known to small pore wall thickness of 1 nm, which is too thin to accommodate those highly developed zeolitic nanoparticles. Although MCM-41 with thicker pore walls of 2.0 nm, as matrix material, was impregnated with TPAOH and further recrystallized by DGC method, MFI zeolite particles were also observed with an increase in the amount of TPAOH and the duration of the DGC process [34]. For this point, SBA-15 with large wall thickness of 3 nm would be a better choice as a mesoporous matrix. However, the hydrothermal treatment of as-made SBA-15 impregnated with TPAOH still led to the mixtures of mesoporous/ZSM-5 composite containing some segregated zeolite crystals in size of 5 nm [21]. Furthermore, another SBA-15 with thicker walls of 6.0 nm was used as parent material, and SBA-15/MFI type materials without any phase separation were obtained by dry-gel conversion after the impregnation of TPAOH [35]. In a word, pore wall thickness, the medium for the recrystallization and the interactions among SDA, surfactant and silica walls all play key roles in the maintenance of the mesostructure during the crystallization process. In this present work, hierarchical MCM-41/MFI composites were synthesized through ion-exchange of as-made MCM-41 type

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mesoporous materials with tetrapropylammonium bromide (TPABr) and subsequent steam-assisted recrystallization, where TPABr was used as SDA instead to decrease the damage of MCM-41 structure caused by the impregnation with TPAOH and hexadecylamine retained in MCM-41 pore retarded the rapid collapse of the mesostructure during recrystallization treatment. This multistep synthesis led to hierarchical porous materials with both an ordered mesostructure and a zeolite MFI phase, where no phase separation occurred. Among others, the presence of Ti species and low re-crystallization temperature could retard the nucleation and growth of crystalline MFI domains, which is beneficial to avoid the complete collapse of the mesoporous structure and the formation of the physical mixtures of amorphous mesoporous material and bulky zeolite. But the exact role of Ti species in this synthesis is still under the way. 2. Experimental procedures 2.1. Synthesis of MCM-41 type mesoporous materials In a typical synthesis, 2.22 g cetyltrimethylammonium tosylate (CTATos, Merck, 99%) was dissolved in 41 ml water and stirred for 1.5 h at 80 8C. 6.95 g tetramethylammonium hydroxide (TMAOH, SCRC, 25%) aqueous solution was added into 32 ml water, followed by the addition of 3.25 g fumed silica under vigorous agitation. The silicate solution was stirred for 2 h at 80 8C and cooled to ice temperature, then tetrabutyl titanate (TBOT, SCRC, 98%) isopropanol solution containing 0.37 g TBOT and 2 g isopropanol was added slowly and continued to stir for 30 min at 0 8C. Alcohol was removed by stirring at 60 8C for 1 h. The above obtained solution was dropwise added into CTATos solution. The collected gel composition was SiO2: 1/x TiO2: 0.09 CTATos: 0.35 TMAOH: 79.9 H2O (x = Si: Ti). The mixture was stirred continuously for 2 h at 80 8C and then loaded into a 100 ml Teflon-lined steel autoclave where the gel mixture was pre-aged for 2 days at 35 8C. The final mixture was heated in autoclave under static condition at 175 8C for 2 days. Solid obtained was fully washed by water, and then was dried at 80 8C in the oven overnight. 2.2. Recrystallization experiments To prepare mesoporous zeolite, 1 g above as-synthesized mesoporous material was added to TPABr (Aldrich, 98%) ethanol solution with the molar ratio of TPABr to CTATos of 2.5. After stirring at 40 8C for 15 min, the mixture was fully washed by ethanol and acetone, dried at 80 8C for 0.5 h and then put in a test tube without the contact with liquid. The test tube and holder was introduced into a 100 ml Teflon-lined steel autoclave containing 1.5 g water and 0.1 g ammonia to produce saturated steam and heated at 175 8C for 1–7 days. After steam-assisted synthesis (drygel conversion), the autoclave was cooled to room temperature and the solid was separated, washed with ethanol and water, air-dried and finally calcined in air at 550 8C for 5 h to ensure the complete removal of the template and other organics. The final obtained samples were labeled as M x-t-y, where x indicated the molar ratio of silica to titanium, t and y meant the recrystallization temperature and particular duration in hours, respectively. 2.3. Characterization Powder X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance powder diffract meter using Cu Ka radiation (l = 0.154184 nm) over a 2u range from 0.508 to 5.08 and 58 to 358, the accelerating voltage and the applied current were 40 kV and 40 mA, respectively. SEM was performed on a scanning electron microscopy (type HITACHI S-4800) with an accelerating voltage of

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3 kV. TEM of as-synthesized samples was collected on a JEM-2010 operating at 200 kV. To prepare the samples for TEM, a dispersion of the sample in diluted ethanol was dropped onto the TEM sample bronze gridding, dried at room temperature for 1 h. FT-IR spectra were recorded on a Nicolet Fourier transform infrared spectrometer (NEXUS 670) using the KBr technique. Solid UV/vis spectra were recorded in the 200–500 nm range on UV2550 from aluminum cell with quartz window. The BET surface area (SBET) and pore parameters of the samples were determined by nitrogen adsorption–desorption measurements at 77 K on a nitrogen adsorption apparatus (BELSORP-max). Before the measurements, the samples were out gassed at 300 8C in vacuum for 6 h. The pore size distributions were derived from the adsorption branches of the isotherms using the Barrett–Joyner–Halanda (BJH) method. The total pore volume (Vp) was estimated at a relative pressure of 0.99. Thermogravimetric (TG) analysis was performed using a PerkinElmer TGA analyzer with a heating rate of 10 8C/min under an air flow. Elemental analyses of CHNS were carried out on a PerkinsElmer 2400 series II CHNS/O analyzer. 1H–13C CP/MAS NMR measurements were collected on a Bruker DSXv400 spectrometer.

Iintensity

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a

b

1

2

3

4

5

6

7

2 Theta (degree) 3. Results and discussion The XRD patterns of parent, as-synthesized M50 and M0 samples are presented in Fig. 1a and b, respectively. There are three well-resolved reflections indexed as (1 0 0), (1 1 0) and (2 0 0), confirming well ordered hexagonal mesoporous structure. Some of us have reported that this is the first MCM-41 with highly ordered structure that can be synthesized at a crystallization temperature above 165 8C using solo hydrocarbon surfactant without any additive in the pure silica system [36]. The introduction of Ti in this work led to almost same meso-structures as shown in Fig. 1a. Most interestingly, the d1 0 0 spacing of as-made sample is 5.4 nm and the pore size distribution (PSD) curves calculated by Barrett– Joyner–Halenda (BJH) method from adsorption branch centered at 3.7 nm. The pore wall thickness (t) of the parent M50 canpbe ffiffiffi roughly deduced from the unit cell parameter (a0 ¼ 2 d1 0 0 = 3) and the mean mesopore diameter (dpore), by t = a0  DBJH, is 2.6 nm, which is much larger than that previously reported by Kresge et al., Corma et al., and Kruk et al. (less than 1.0 nm) [2,37,38]. The thick walls of these amorphous mesoporous precursor material is very important for the maintenance of the ordered mesostructure during the recrystallization process. TPA+-exchanged M50 retained well ordered hexagonal mesostructure and showed a strong increase in the intensity of the d1 0 0 signal due to the partial removal of surfactant molecular, confirming that the ion-exchange using TPABr could avoid severe damages of ordered MCM-41-like

Fig. 1. Small angle X-ray diffraction spectrums of as-prepared (a) M50 (Si/Ti = 50) and (b) M0 (Si/Ti = 1, free metal) materials synthesized at 175 8C for 2 d.

structure caused by high basicity when TPAOH was used instead. Moreover, higher synthesis temperature may lead to more condensation of silica walls and thus less surface silanol groups, contributing to the improvements of the thermal, hydrothermal and mechanical stability of this mesoporous material. SEM image of as-made M50 sample in Fig. 2a exhibits loose particles in size of about 0.5–2 mm along with some larger elongated agglomerates, which is smaller than that of pure silica M0 sample as shown in Fig. 2b. In this work, compared with the results reported by Zhang et al. [36], higher pre-heated temperature was chosen before the aging at 35 8C to make complete hydrolysis of fumed silica thus avoid the formation of some small silica particles in the parent mesoporous materials. The XRD patterns of the materials M50-175-y obtained after dry-gel conversion at 175 8C were shown in Fig. 3. All the samples preserved only a single well-defined (1 0 0) peak in low-angle XRD patterns with obvious decrease in the intensity after the DGC process, indicating less ordered hexagonal pore arrangement due to the partial consumption of parent M50 under the DGC conditions. Furthermore, some reflections in 2u range of 22–258 could be observed in wide-angle XRD patterns, which could ascribe to zeolite nanoparticles with MFI structure formed in these

Fig. 2. Scanning electron micrograph of as made MCM-41 (a) M50 (Si/Ti = 50), (b) M0 (Si/Ti = 1, free metal) obtained using CTATos as template at a synthesis temperature of 175 8C for 2 d (see Section 2).

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X 10

d Intensity

Intensity

d c

c b

b

a

a 1

2

3

4

5

6

7

5

10

15

20

25

30

35

40

2 Theta (degree)

2 Theta (degree)

Fig. 3. Low (left) and wide (right) angle XRD patterns of M50 composite materials after recrystallized at 175 8C for (a) 24 h, (b) 48 h, (c) 72 h, and (d) 168 h, respectively.

mesoporous composites. These reflections with low intensities and considerable line broadening indicated that no well crystallized zeolite phases formed. The formation of zeolite was also proved by FT-IR spectra (Fig. 4), in which the M50-175-48 showed an absorption band at 561/547 cm1 (doublet) (Fig. 4d), which was not present in the parent mesoporous materials or TPA+ exchanged sample (Fig. 4a and b). This band around 550 cm1 has been assigned to the asymmetric stretching mode in five-membered ring blocks and the splitting of this lattice-sensitive band into a doublet at 561/547 cm1 is characteristics of nano-crystals of the MFI structure (TS-1) [39]. With the recrystallization duration increasing, the recrystallized M0-175-y samples showed enhance diffraction peaks in 2u range of 22–258 and the (1 0 0) reflection in low-angle XRD patterns gradually decreased, as shown in Fig. 5. In case of the recrystallized M50 samples, reflections in 2u range of 22–258 kept almost same with the re-crystallization duration increasing, while (1 0 0) reflection has no remarkable decrease as shown in Fig. 3,

a b c

Transmittance (a.u.)

d e

1000

900

800

700

600

Wavenu mbe r / cm

500

400

-1

Fig. 4. FT-IR spectra of (a) M50, (b) TPA-M50 obtained by ion-exchange procedure, (c) M50-175-24, (d) M50-175-48, and (e) M50-175-168.

indicating that the mesostructure of M50 parent sample remains better during the recrystallization. These results suggest that the introduction of titanium obviously inhibit the crystallization of MFI phase and well kept the mesostructure. In a word, during DGC process, initially amorphous walls of the mesoporous material are progressively transformed into zeolite nanoparticles, and the formation of zeolite nano-crystallites is accompanied by partial collapse of the MCM-41-like mesostructure. As shown in Fig. 6a, the recrystallized M50 after the dry-gel conversion at 175 8C for 2 days showed similar size and morphology with that of the parent M50 in Fig. 2. Most elongated or rod-like mesoporous particles did not exhibit the distinct morphological change during the high-temperature DGC treatment, indicating the absence of extensive restructuring. No presence of separated MFI zeolites in M50-175-48 sample implied that no extensive restructuring of MCM-41 and dual templates may work in a cooperative manner. On the contrary, separated MFI particles were observed in purely siliceous M0-175-48 sample without the introduction of titanium. Furthermore, this process went even faster, resulting in silicalite-1 [24], and M0-175-48 sample showed much higher relative crystallinity of MFI structure than M50-175-48, as shown in Figs. 3 and 5. Both the results of XRD and SEM indicated that the presence of titanium severely retarded the crystallization process, in favor of the maintenance morphology of mesostructure of parent M50. TEM is the ultimate technique for determining whether a combined zeolitic/mesoporous material is a composite structure with separate zeolitic phases and mesoporous parts or a ‘‘true’’ hierarchical mesoporous material with zeolitic walls and it is proved a powerful technique for the investigation of genesis of nano-zeolites in the mesoporous silica matrix [40,41]. As shown in Fig. 7a, the partial recrystallization of M50 into the MFI nanodomains brought about changes in the walls’ thickness and a small decrease in the pores regularity though the novel material has retained its 2D hexagonal mesoporous character, presenting a remarkable degree of order as shown by Fourier diffractograms (inset). The boundary between two-dimensional channels cannot be easily distinguished in the sample recrystallized at 175 8C for 48 h due to the formation of the zeolite nano-particals in mesoporous channels, significantly differing from that parent MCM-41 materials which exhibits the typical 2D channels with pore size of 3.0 nm. In addition, some hollow defects could be observed in the MCM-41 particles, similar to that reported by Lin

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X 10

d

d

Intensity

Intensity

c b

c

b

a a

1

2

3

4

5

6

7

2 Theta (degree)

10

15

20

25

30

35

40

2 Theta (degree)

Fig. 5. Low (left) and wide (right) angle XRD patterns of the M0 composite materials after recrystallized at 175 8C for (a) 12 h, (b) 24 h, (c) 36 h, and (d) 48 h, respectively.

et al. [41]. This may attribute to local desilication caused by alkaline attack during DGC process. Furthermore, zeolitic nanodomains in size of 10 nm can both located at the edges of the mesoporous regions (in the black circles) and embedded in the mesoporous wall (in the white circles), as shown in Fig. 7b. TEM image taken perpendicularly to the mesopore walls shows microporous framework within the mesopore walls, indicating that some domains of the mesopore walls possess a zeolitic

crystalline microporous framework. Similarly, the ZSM-5 nanodomains could be spherical particles with diameters of approximately 28 A˚ estimated by de Moor et al. [42] using small-angle Xray scattering or slabs with dimensions of 13 A˚  40 A˚  40 A˚ suggested by Kirschhock et al. [43]. As it is suggested that the mesoporous materials with thicker walls may effectively sustain more local bond strains and accommodate zeolitic nano-crystals to the meso-structured walls [44], M50 and M0, with thick walls is up

Fig. 6. SEM images of samples: (a) M50-175-48 and (b) M0-175-48 samples.

Fig. 7. HR-TEM images of composite M50-175-48 with (a) mesoporous region taken with an incidence perpendicular to the direction of the mesopores and (b) zeolitic particles at the edges of the mesoporous part. Insets are Fourier diffractograms.

H.L. Chen et al. / Materials Research Bulletin 47 (2012) 1774–1782

to 2.5 nm, could be better choice for parent materials, which accommodate zeolitic nano-domains and well maintain the mesostructure during the recrystallization, as proved by this study. However, Jacobs et al. [45] considered that a crystal size of approximately 100 A˚ is required for the detection of crystalline MFI structure by X-ray diffraction, which is much larger than the thickness of pore walls. There are two possible reasons for the XRD results in this work. Firstly, zeolitic nano-particles embedded in the mesoporous wall may develop accompanied by the breakdown of amorphous pore walls and ordered mesostructure, and zeolitic nanoparticles may span several pores but not segregate from the mesoporous particles. Secondly, zeolitic particles detected at the edges of the mesoporous regions are in size of larger than 100 A˚

a b c

Adsorption (a.u.)

and it could attribute to the recrystallization of amorphous walls, which was directed by TPA+ deposited at the mouth of mesopore. Similar result was reported by Vernimmen et al. [46], where a combined zeolitic/mesoporous composite material was obtained and zeolitic particles can be detected at the edges of the mesoporous regions. Both of these two kinds of zeolite particles could contribute the exhibition of strong peaks in wide-angle XRD patterns. Moreover, DR UV–vis spectra of samples (Fig. 8) before and after the recrystallization showed that no obviously TiO2 phases were present in these materials even after the recrystallization at 175 8C for 7 days, as no absorption was detected at 330 nm. These samples exhibited an absorption maximum below 250 nm, usually assigned to Ti atoms tetrahedrally coordinated to the silica framework. This could be beneficial for redox reactions with large molecules that require tetrahedrally coordinated titanium. Nevertheless, some slight absorption was observed at higher wavelengths, indicating the presence of Ti species with octahedral coordination. This is also possibly caused by asynchronous incorporation of silicon and titanium species during the recrystallization procedure [47]. For the system with no titanium added, homogenous MCM-41/ MFI composite could be also obtained after the recrystallization at 150 8C for 48 h. As shown in the XRD patterns of Fig. 9, TPA+-exchanged M0 samples recrystallized at 150 8C show similar trend toward M50-175-y samples. Furthermore, SEM images (Fig. 10) showed no presence of any separated MFI particles, confirming that the extensive restructuring of MCM-41 were

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200

300

400

500

600

700

800

Wavelength (nm) Fig. 8. UV–vis spectra of (a) as-made M50 and products obtained from dry-gel conversion at 175 8C for (b) 48 h, and (c) 168 h using as made M50 as parent material.

avoided during DGC process at 150 8C. This result was quite different from the M0 samples recrystallized at 175 8C, which indicated that lower temperature is also beneficial to keep the mesostructure through reducing growth rate of MFI zeolite nanodomains during the recrystallization process. Besides, the present approach to position the appropriate amount of template molecules inside mesoporous materials is highly important for the controlled partial recrystallization. When the pre-positioning of TPA+ was omitted and a homogeneous aqueous mixture of TPAOH and MCM-41 was heated, the MCM-41 was recrystallized overnight into aggregates of MFI structured zeolite. Thus, the preposition of TPA+ at the interporous surface of the mesoporous material is a pre-requisite to well keep the mesostructure of parent materials during the recrystallization process. Furthermore, ion-exchange method was used instead of

d

Intensity

Intensity

d

c

c

b

b

a

a

1

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3

4

2 Theta(degree)

5

6

7

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15

20

25

30

35

40

2 Theta(degree)

Fig. 9. Low (left) and wide (right) XRD patterns of the M0 composites materials after recrystallized at 150 8C for (a) 24 h, (b) 48 h, (c) 72 h, and (d) 96 h, respectively.

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Fig. 10. SEM images of recrystallized M0-150-48 samples obtained from dry-gel conversion at 175 8C for 48 h.

impregnation to introduce TPA+ into channels of parent materials, minimizing the position of TPA+ on the external surface. And TPABr was chosen as a template instead of TPAOH in an effort to avoid serious damage of the MCM-41 material caused by high basicity. Both of them will play key roles in the preservation of mesostructure during recrystallization. According to the results from TG analysis, the total weight loss of as-made M50 and TPAM50 was 42.7% and 29.6%, respectively. This decrease in weight loss indicated that some surfactants were removed out of mesoporous channels during ion-exchange process. Furthermore, carbon/nitrogen (denoted as C/N) molar ratio also decreased in the process of ion-exchange, from 16 to 14, further proving the removal of surfactant molecules out of mesopores. Therefore, TPA+, as a charge compensator, was successfully introduced into the parent materials by ion-exchange. Most importantly, remained CTA+ in the channel of TPA-MCM-50 may play a key role in supporting the mesostructure. A powerful proof of the presence of CTA+ in the mesopore channels is from 1H–13C CP/MAS NMR spectrum. As shown in Fig. 11, before and after recrystallization, the amount and structure of organic species did not change a lot, indicating the absence of complete decomposition of CTA+ during dry-gel conversion. These existence of CTA+ could support the mesopore channels thus avoid the strict collapse of the mesostructure in recrystallization process. Nitrogen physisorption results give the textual properties of these samples. As shown in Fig. 12, the parent M50 or M0 samples exhibited type IV isotherms with a sharp inflection point at relative pressure 0.30–0.60 due to the capillary condensation, which is the characteristics of mesoporous materials [48]. The adsorption– desorption isotherms of M0 sample has a type H1 hysteresis loop, while those of M50 shows some characteristics of H2 hysteresis loop. This could attribute to less mesoscopic ordering due to the introduction of titanium. These initial mesoporous materials

Fig. 11. 1H–13C CP/MAS NMR spectra of samples (a) TPA-M50 and (b) M50-175-48 obtained from ion-exchange and dry-gel conversion at 175 8C for 48 h, respectively.

exhibited a narrow pore size distribution centered at 4.1 and 3.7 nm calculated using BJH method from adsorption branch for M0 and M50, respectively. After the recrystallization for 48 h by DGC, the isotherms and hysteresis loop changed a lot. M50-175-48 shows the isotherms with combined characteristics of type I and IV, with the former being typical for microporous materials and later for mesoporous materials [49]. The hysteresis loop varied from H2 to H4, indicating the partial restructuring during DGC process. Importantly, pore size distribution of sample M50-175-48 is also centered at about 3.7 nm, which means the maintenance of mesostructure of parent M50 during DGC process. However, the distribution of the mesopore diameter for sample M50-175-48 is much broader than for that of the parent M50, indicating partial collapse of the M50 mesoporous structure after the recrystallization for longer time, which are in good agreement with the XRD results. Moreover, their adsorption branch was shifted toward lower relative pressure and the mesopore shrink slightly to smaller diameter upon the recrystallization, which might be explained by the formation of the zeolite crystals in mesopore channels. Furthermore, the recrystallization leads to a decrease in SBET and Vmeso but an increase in Vmicro. As listed in Table 1, SBET, Vt and Vmeso of the recrystallized samples are markedly smaller than those of

Table 1 Influence of crystallization temperature and substituted heteroatom on the texture properties of calcined products. No

Si/Ti

T (8C)

Crystal. time (h)

SBET (m2/g)

Vt (cm3/g)

Vmia (cm3/g)

DBJHb size(nm)

Tc (nm)

1 2 3 4 5

50 50 1 1 1

– 175 – 150 175

– 48 – 48 48

609 254 510 432 357

0.77 0.44 0.68 0.60 0.44

0 0.03 0 0.05 0.05

3.7 3.7 4.1 3.3 3.3

2.6 – 2.8 – –

a b c

Microporous volume calculated by t-plot. calculated from adsorption branch using BJH method. pffiffiffi Pore wall thickness= 2 d1 0 0 = 3  DBJH .

b

0.6

M50 400

M50 M50-175-48

0.5

300 200

M50-175-48

0.4 0.3 0.2

100

0.1

0 0.0

0.0

0.2

0.4

0.6

0.8

1.0

2

Relative pressure (p/p ) 0 500

4

6

8

0.6

d

0.5

400 M0 M0-150-48

200

M0 M0-150-48 M0-175-48

0.4

dVp/d(dp)

300

10 12 14

Pore size (nm)

c

3 -1

Volume adsorbed (cm g STP)

1781

0.7

a

500

dVp/d(dp)

Volume adsorbed (cm3g-1 STP)

H.L. Chen et al. / Materials Research Bulletin 47 (2012) 1774–1782

M0-175-48

0.3 0.2 0.1

100

0.0

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (p/p0)

2

4

6

8

10 12 14

Pore size (nm)

Fig. 12. Nitrogen adsorption–desorption isotherms (a and c) and pore distribution curves (b and d) of porous samples.

the parent MCM-41 materials, attributing to the partial consumption of mesostructure, while Vmicro increases with the formation of crystalline MFI domains correspondingly. Similar results were observed when pure silica MCM-41 was recrystallized at 150 8C, however, if it was treated at higher temperature, Vt and Vmeso obviously decreased due to the formation of bulky zeolite thus complete consumption of mesostructure. All results proved that the sample M50-175-48 is a zeolitic/ mesoporous composite material, which is quite different from physical mixture of mesoporous material and zeolite, making them very valuable in sorption, separation and catalytic applications. 4. Conclusions In summary, highly ordered 2D hexagonal meso-structured porous silica with thicker pore wall up to 2.5 nm as parent mesoporous materials were synthesized using solo hydrocarbon surfactants (CTATos) as templates, and titanium were successfully introduced during the synthesis process of parent materials. The mesoporous precursor with amorphous walls provides a starting material from which nano-crystalline zeolite domains can nucleate and grow. The mesoporous zeolite can be fabricated from assynthesized MCM-41 with or without titanium through dry-gel conversion method, before that, small structure-directing agent

(SDA), TPA+ cations, were positioned at the interporous surface of parent mesoporous material by ion-exchange method in order to achieve a cooperative templating effect of both mesoscale surfactant and small SDA during the recrystallization process. Final recrystallization was performed in the water-vapor phase, where less water contacted with silica walls, inhibiting the easy transportation of silica species from and to and thus better maintaining the mesostructure. Importantly, the introduction of hetero-atom, titanium, was investigated. The recrystallized materials retained their mesoporous characteristics, as evidenced by TEM images, low-angle XRD patterns and nitrogen sorption data. Moreover, MFI structure appeared in wide-angle XRD patterns led to no presence of separated MFI particles when titanium was introduced and lower recrystallization temperature was chosen. In a word, the parent materials with thicker pore walls, the presence of titanium (IV), the preposition of TPA+ by ionexchange and lower temperature are all important for maintaining the mesostructure of parent mesoporous materials thus avoid the complete collapse of the mesoporous structure or the formation of the physical mixtures of amorphous mesoporous material and bulky zeolite. Such a synthesis principle could be applied to prepare other hierarchically structured materials, using different parent mesoporous materials and different SDAs that direct other zeolites.

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